(NewsNation) — Migrants with criminal records could be making it to the U.S. undetected thanks to a spotty vetting process.

U.S. Border Patrol told NewsNation that in October alone at least 64,000 undocumented immigrants evaded law enforcement. Those are people who were seen on camera or during a pursuit who are never apprehended. Since the migrants aren’t apprehended, they don’t go through the vetting process.

In the Tuscon sector last week, three Mexican nationals with criminal convictions for sexual assault in the U.S. were arrested among groups trying to remain undetected.

But even when migrants self-surrender, criminal histories may not be discovered.

The criminal background check system only detects crimes committed in the U.S. and not those committed in other countries.

“The system is able to see if you’ve committed a crime in the United States. But if you’ve committed a crime in your own country, your record is going to be squeaky clean here,” said National Border Patrol Council Vice President Art del Cueto.

A Senate panel asked Homeland Security Secretary Alejandro Mayorkas how many immigrants with criminal backgrounds make it to the U.S. and he was unable to answer.

Trac Migration found 31 percent of migrants in ICE detention facilities have some sort of criminal background here in the U.S. but does not have data on crimes committed in other countries.